1. Field of the Invention
The present invention relates to a thermal control apparatus for applying simultaneous thermal control to a plurality of objects using heaters or the like, a space craft provided with said thermal control apparatus, and a thermal control method. For example, the present invention relates to thermal control for an artificial satellite, a space station, or a space craft such as a transporter for space navigation.
2. Related Art
FIG. 6 schematically shows a conventional thermal control system for an artificial satellite. The system includes a power source 1, and a calculator 6 for determining/instructing ON/OFF of heaters 3, as will be described below. The calculator 6 includes a heater ON/OFF determining circuit 7 for the heaters 3. A plurality of thermal control objects 21, 22, 23 . . . 2i . . . (hereinafter will be referred to as xe2x80x9cobjectsxe2x80x9d) are installed in an artificial satellite. For each of the objects 2 is provided a heater 31, 32, 33 . . . 31, . . . , a temperature sensor 41, 42, 43 . . . 4i . . . , and a switch 51, 52, 53 . . . 5i . . . for controlling ON/OFF of the heater 3 according to instructions from the calculator 6. In the drawing, numerals 2xcx9c5 have subscripts 1, 2, 3, . . . i . . . in this order from the top to indicate that a plurality of elements are provided. For an artificial satellite, the object 2 may include, for example, an electronic device for attitude control, communication, data processing, power supply or the like, a propeller device, and an antenna device, which are disposed in the satellite structure.
The operation of a conventional thermal control system for artificial satellites will be now described with reference to FIGS. 6 and 7. In this related art system, ON/OFF control for the heaters 3 is performed in the heater ON/OFF determining circuit 7 based on the logic depicted in FIG. 7.
First, at step S1, the temperature Ti of each object 2i is measured using the temperature sensor 4i mounted on the object 2i. At step S2, the temperature Ti is compared with the lower limit control temperature Tia of the object 2i. When Ti less than Tia, the calculator 6 instructs the switch 5i to be ON according to step S3, so that the heater 3i is turned on to raise the temperature Ti. When Tixe2x89xa7Tia at step S2, the temperature Ti is further compared with the upper limit control temperature Tib of the object 2i at step 4. When Tixe2x89xa7Tib, the calculator 6 instructs the switch 5i to be OFF according to step S5, such that the heater 3i is turned off to thereby reduce the temperature Ti of the object 2i. When Ti less than Tib at step S4, on the other hand, the current ON/OFF status for the heater is maintained (this operating condition is referred to as xe2x80x9chysteresisxe2x80x9d). According to the above-described thermal control of the related art, the temperature Ti of the object 2i is controlled within a range approximately between Tia and Tib, as shown in FIG. 8.
In the above-described thermal control system for artificial satellites, when thermal control is simultaneously applied to a plurality of objects, the heaters consume power as shown in FIG. 9. FIG. 9 illustrates an example when three objects are thermally controlled by means of three heaters, respectively. The average power of the three heaters is indicated by a dotted line. In this example, there is a possibility that all three heaters are simultaneously turned on, in which case the peak power consumption by the heaters is far greater than the average power. In view of balance of heat, thermal control should be possible as long as power corresponding to the average power is continuously consumed by the heaters. In the conventional method, however, a significantly large peak power (maximum power) is produced by simultaneously actuating a plurality of heaters, which adversely affects a power system of the artificial satellite.
For example, more than a few of the artificial satellites in operation today actually have 100 or more heaters installed therein. In these artificial satellites, peak power consumption by the heaters reaches several to ten times the average power thereof and therefore cannot be disregarded when compared with power required for various devices installed on the satellite. Accordingly, the power system of the satellite must be designed so that it can supply power sufficient to cover such peak periods, but doing so introduces significant disadvantages regarding weight or cost of the satellite resulting from including such a power source with large maximum supply capacity.
The present invention was made so as to solve the above problems of the conventional thermal control for an artificial satellite or the like, and aims to reduce the peak energy (maximum energy) which must be supplied to heaters or coolers, which is produced when a plurality of heaters and coolers are simultaneously actuated.
To this end, in accordance with one aspect of the present invention, there is provided a thermal control apparatus comprising a plurality of heaters and/or coolers each controlled to be ON and OFF for changing a temperature of each of a plurality of thermal control objects; a plurality of temperature sensors each provided in each of said plurality of thermal control objects for measuring the temperature of said thermal control object and outputting the measured temperature; a heater and/or cooler ON/OFF priority reading table which, in response to input of said measured temperature, outputs priority for turning on said plurality of heaters and/or coolers, based on order values for turning on said plurality of heaters and/or coolers, predetermined in correspondence with temperature, and said measured temperature; and an energy managing section for turning on the heater or the cooler with high priority and turning off the heater or the cooler with low priority based on said priority output from said ON/OFF priority reading table, such that a sum of energy supplied for actuating said plurality of heaters or coolers does not exceed an energy limit.
In accordance with a second aspect of the present invention, each of said order values is determined for every plurality of temperature levels within a temperature control range established for each of said plurality of thermal control objects.
Further, in accordance with a third aspect of the present invention, said priority is determined based on the speed of temperature response of said plurality of thermal control objects.
Still further, in accordance with a fourth aspect of the present invention, higher priority is assigned to a heater or cooler of a thermal control object with higher speed of temperature response and to a heater or cooler of a thermal control object whose measured temperature is more approximate to a lower limit.
In accordance with a fifth aspect of the present invention, said temperature control range includes a level of upper limit approximate temperature adjoining the upper limit of said temperature control range, and when said measured temperature falls within said level of upper limit approximate temperature, the heater or the cooler provided in the thermal control object having said measured temperature is turned off.
In accordance with a sixth aspect of the present invention, said temperature control range includes a level of OFF maintaining temperature, such that, when said measured temperature falls within said level of OFF maintaining temperature and a heater or cooler provided in the thermal control object having said measured temperature is in an off state, the heater or cooler is controlled to be maintained in an off state.
In accordance with a seventh aspect of the present invention, the upper limit of said level of OFF maintaining temperature adjoins the lower limit of said level of upper limit approximate temperature.
In accordance with an eighth aspect of the present invention, said energy limit is set to be slightly greater than the average energy level for said heaters or coolers in operation.
In accordance with yet another aspect of the present invention, there is provided a space craft provided with said thermal control apparatus for thermally controlling a plurality of thermal control objects.
In accordance with still another aspect of the present invention, there is provided a thermal control method for controlling a plurality of heaters and/or coolers controlled to be ON and OFF for changing the temperature of each of the thermal control object, the temperature of each of said plurality of thermal control objects being measured by one of a plurality of sensors, said thermal control method comprising a first step of determining priority for turning on said heaters or coolers based on said measured temperature; a second step of sequentially adding energy supplied for turning the heater or the cooler on, in the order starting from a heater or a cooler with higher priority of the heaters or the coolers whose priority is determined in said first step; and a third step of sequentially comparing the sum of energy sequentially added in said second step with an energy limit and, when the sum of energy exceeds said energy limit, turning on the heaters or the coolers corresponding to the energy which was added immediately before the time when the energy limit is exceeded, wherein said first, second, and third steps are repeatedly executed.
According to the present invention as described above, it is possible to reduce the level of the peak energy supplied from an energy supply system to heaters and/or coolers.